1. Field of the Invention
The invention relates to a frictional hinge device used to pivotably support various lid plates at desired angular positions including opening and closing positions, and concerns to a portable business machine such as a laptop note type personal computer into which the frictional hinge device is incorporated to hold a display at the desired angular positions.
2. Description of Prior Art
In this type of the frictional hinge device, a lid plate is secured to a metallic shaft which is pivotably supported on a holder block metal. The holder block metal clamps the metallic shaft to produce a surface frictional resistance between the holder block metal and the metallic shaft so as to hold the lid plate at desired angular positions.
Although this makes a whole structure simple and contributes to cost reduction, a certain quantity of grease is required for lubrication between the holder block metal and the metallic shaft. The grease becomes a likely cause of perimetric pollution around the frictional hinge device.
In order to avoid these inconveniences, a shaft lock device is disclosed by Laid-open Japanese Patent Application No. 7-26825 (laid-open on Jan. 27, 1995, assigned to Kabushiki Kaisha Kato Spring Seisakusho). This laid-open publication teaches that an outer shaft is made of a synthetic resin and integrally molded with an inner shaft. Due to a surface frictional resistance caused from a thermal expansional difference between the inner and outer shafts, the inner shaft is held at any desired angular positions while permitting the inner shaft to pivot relative to the outer shaft against the force of the surface frictional resistance therebetween. Due to the surface frictional resistance, a display is held at the desired angular positions in a laptop note type personal computer.
In the shaft lock device disclosed by the Laid-open Japanese Patent Application No. 7-26825, providing a surface roughness, surface treatment and frictional coefficient are suggested as means to determine the frictional torque between the inner and outer shafts together with their diametrical dimensions.
However, this disclosure remains silent about qualitative and quantitative analyses on a relationship between the inner and outer shafts. This causes no smaller variations on the frictional torque when the inner shaft pivotally moves relative to the outer shaft. This also causes abnormal noise due to a stickslip phenomenon when pivotally moving the inner shaft, thereby losing a good endurance with a reduced frictional torque due to an unacceptable amount of wear between the inner and outer shafts.
Therefore, the present invention has been made with the above drawbacks in mind.
It is a main object of the invention to provide a frictional hinge device which is inexpensive with no fear for perimetric grease pollution and no abnormal noise accompanied with a stickslip phenomenon with the least torque variations, and is capable of maintaining a stable surface frictional resistance between a shaft member and a support member for an extended period of time so as to repeatedly hold the support member at desired angular positions based on a substantially uniform surface frictional resistance.
With a frictional hinge device having a support member rotatably supported by a shaft member, a support member is integrally molded around the shaft member when a synthetic resin is injected into a mold die in which the shaft member is placed beforehand.
Due to the synthetic resin contracted by a residual stress caused from a shrinkage allowance when solidified, the support member tightly engages with the shaft member. This provides a good surface frictional resistance therebetween. When the support member is subjected to a frictional torque greater than the surface frictional resistance, the support member pivots around the shaft member relatively. When the support member is subjected to a frictional torque less than the surface frictional resistance, the support member is held at an appropriate angular position by the surface frictional resistance.
With the synthetic resin molded around the shaft member, the support member and the shaft member are assembled quickly with the least manufacturing cost.
Thus, the support member tightly engages with the shaft member due to the synthetic resin contracted by the residual stress caused from the shrinkage allowance when solidified, and a strain appears within the synthetic resin of the support member in correspondence to the residual stress.
The inventors carried out experimental tests by paying their attention to a relationship between a strain distribution and a torque holding rate. As a result, the inventors found that the torque holding rate falls rapidly to deteriorate the endurance when the strain distribution comes inequable such a degree as to exceed 15% (referred to as xe2x80x9cinequable strain distribution degreexe2x80x9d hereinafter).
The torque holding rate T (%) is expressed by the formula below.
T (%)=(a torque measured after undergoing a heat deteriorating experimental test or an endurance experimental test)xc3x97100/(an initial torque) 
The strain distribution within the synthetic resin is equalized so that the inequable strain distribution degree is 15% or less. Where the inequable strain distribution degree (%) is expressed by (|maximum strain (minimum strain)xe2x88x92average strain|)xc3x97100/(average strain) in which a greater one is selected when compared the absolute value |maximum strain| with the absolute value |minimum strain|.
In order to realize these requirements, the support member is quality controlled based on a molding method, configuration, post-treatment and molding conditions to produce a frictional hinge device superior in endurance.
With the high and stable torque holding rate thus achieved, a smoothness is imparted to the shaft member to avoid an unfavorable coagulation against the synthetic resin to obtain an appropriate frictional resistance between the shaft member and synthetic resin. This also reduces frictional torque variations and a stickslip phenomenon significantly with no abnormal noise induced due to the stickslip phenomenon when the support member pivots relative to the shaft member.
In order to equalize the strain distribution within the synthetic resin, the synthetic resin is partly thickened or partly thinned.
In order to also equalize the strain distribution within the synthetic resin, a film injection gate or a multiple point injection gate is provided, the former of which flows the synthetic resin smoothly and the latter of which flows the synthetic resin dispersively when the synthetic resin is molded around the shaft member.
In order to further equalize the strain distribution within the synthetic resin, the synthetic resin is dealt with a heat treatment at a temperature of 0.8xc3x97Tg (xc2x0C.) or higher after the synthetic resin is molded around the shaft member. Where Tg (xc2x0C.) is a vitreous transformation temperature when the synthetic resin metamorphoses into a rubberized property.
In order to furthermore equalize the strain distribution in the synthetic resin, an equable control means is provided to determine mold conditions so as to equally flow the synthetic resin around the shaft member.
With at least two means combined among (a)xcx9c(e) below, the strain distribution within the synthetic resin is synergistically equalized.
(a) partly thickening or thinning the synthetic resin,
(b) providing the film injection gate,
(c) providing the multiple point injection gate,
(d) dealing with the heat treatment at temperatures of 0.8xc3x97Tg
(xc2x0C.) or higher, and
(e) determining molding conditions to equally flowing the synthetic resin around the shaft member due to the equable control means.
With the frictional hinge device used to pivotably move a display for a portable business machine, it is possible to hold the display repeatedly at any desired angular position for an extended period of time while insuring a stable frictional torque with the least amount of wear between the shaft member and the support member.